1. Field of the Invention
The present invention is related to the paper-making industry, and in particular the recycled paper sector. More specifically, a new procedure is described for treating paper pulp using an enzyme composition which allows, most notably, good machining properties of the pulp during the manufacture of paper.
2. Discussion of the Background
The paper-making industry makes increasing use of recycled paper. With regard to the manufacture of corrugated cardboard, for example, increasing use is being made of recycled fiber-based raw materials, and at the same time, the number of recycling processes is increasing. During each of these recycling operations, however, the quality of the raw material deteriorates. To restore a satisfactory level of mechanical properties, the pulp is generally treated in an aqueous suspension, thereby causing machinability problems, especially those related to the water drainage properties of the paper-making composition.
Pulps in aqueous suspensions ready to be used on a paper-making machine may be characterized by various parameters. An especially important property is the SR value, with respect to the expectation that the pulp will possess water-drainage properties. The Schopper-Riegler level (SR) of a pulp is defined as a factor for evaluating the quality of the pulp for the manufacture of paper. It is an expression of the capacity of water to become separated from the suspension under conditions specified in Standard NFQ 50 003. On a scale of 0 to 100, a high SR value signifies a low drainage rate of the suspension, while a low value signifies a faster drainage rate. It has been observed, for example, that the SR of a pulp which has been subjected to a treatment operation increases to a greater or lesser extent, depending on the degree of treatment undergone, in comparison with a pulp which has not been subjected to an operation of this kind.
This parameter plays an especially important role in the manufacturing capacity of the paper-making machine. Indeed, to increase this capacity, the water-drainage phase must take the shortest possible time. When using suspensions possessing a high SR level, the concentration of pulp must be increased in order to maintain a given rate of manufacture. This phenomenon leads to inferior sheet formation, since the fibers are distributed with greater difficulty. A special advantage is gained, therefore, by using suspensions having a relatively low SR value.
As soon as the suspensions possess an SR level greater than 25, it becomes desirable to attempt to lower it to improve the conditions under which the paper is made. This decrease is made desirable by two considerations. First, the productivity of the paper-making machine can be improved because of accelerated drainage, and second, the manufacturing rate can be preserved without being forced to compensate for the slowness of drainage by reducing the dilution of the suspension, a step which could lead to poor sheet formation.
European Patent No. 0 262 040 describes a procedure for the treatment of paper pulps in aqueous suspensions, especially recycled fiber-based pulps having an SR level of at least 25 which, through the use of enzyme compositions containing cellulases and/or hemicellulases, allows a reduction of the SR and thus an improved drainage of the suspension and improved manufacturing capacity of the paper-making machine.
Enzyme preparations containing cellulases and/or hemicellulases as described are preferably those which possess a C.sub.1 activity, a C.sub.x activity, and a xylanase activity. These three activities are defined by the international enzyme nomenclature and may be designated and expressed as units of the international system per milligram of powder of the enzyme composition in question. The C.sub.1 activity is the activity of the cellobiohydrolase, which may be measured on pure, highly structured cellulose. This activity is demonstrated by the production of cellobiose, and the International System has chosen AVICEL.RTM. as the reference substrate. The C.sub.x activity is measured on modified cellulose, i.e., carboxymethylcellulose (CMC), and is quantified by a decrease in the viscosity of the carboxymethylcellulose or an increase in reducing sugars. The xylanase activity makes possible the hydrolysis of the linking xylans.
As described in the patent cited above, the reactive medium may be more or less suited to the action of the enzymes. Temperature and pH conditions are set to avoiding any risk of denaturation of the enzymes by the medium. The pH thus ranges advantageously between 3 and 7, and the temperature between 20.degree. and 60.degree. C. Above 60.degree. C. the medium tends to denature the enzyme, and below 20.degree. C. the action of the enzymes is produced especially slowly. Using the enzyme compositions described, it is recommended that the pH range between approximately 3 and 7.
Some paper-making recycled fiber-based compositions may possess a high pH which exceeds 7. In this case, it may be necessary to reduce the pH in order to achieve optimal conditions for action of the enzyme compositions.
The reduction of the pH is normally achieved by adding acid, such as sulfuric acid. In addition to the disadvantage of this additional operation, adding an acid may cause disturbances in sheet formation on the paper-making machine, e.g., the reduced effectiveness of certain agents such as retention agents.
Furthermore, in the case of printing and writing papers, for example, a large quantity of acid must be added, thereby decreasing the economic advantage of the enzyme treatment procedure.